The invention relates to a method for operating a working machine and a working machine.
The invention is applicable on working machines within the field of industrial construction machines, in particular wheel loaders. Thus, the invention will be described with respect to a wheel loader. However, the invention is by no means limited to a particular working machine. On the contrary, the invention may be used in a plurality of heavy working machines, e.g. articulated haulers, trucks, bulldozers and excavators.
Wheel loaders are generally provided with an internal combustion engine, a transmission line, a gearbox, driving wheels and a working hydraulic system.
The combustion engine provides power to the different functions of the wheel loader. In particular, the combustion engine provides power to the transmission line and to the working hydraulic system of the wheel loader.
The transmission line transfers torque from the combustion engine to the gearbox, which in turn provides torque to the driving wheels of the loader. In particular, the gearbox provides different gear ratios for varying the speed of the driving wheels and for changing between forward and backward driving direction of the wheels.
The working hydraulic system is used for lifting operations and/or for steering the wheel loader. For this purpose there are at least one hydraulic working cylinder arranged in the wheel loader for lifting and lowering a lifting arm unit, on which a bucket or other type of attachment or working tool is mounted for example forks. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front and rear body part of the wheel loader.
To protect the combustion engine of a wheel loader from rapid changes in the working conditions of the gearbox and the driving wheels it is common to provide the transmission line with a hydrodynamic torque converter or similar arranged between the combustion engine and the gearbox. The hydrodynamic torque converter provides an elasticity that enables a very quick adaptation of the output torque to the changes in the working conditions of the gearbox and the driving wheels of the loader. In addition, the torque converter provides an increased torque during particularly heavy working operations, e.g. during acceleration of the loader. However, these advantages are paid by high losses, since the elasticity and the increased torque provided by the torque converter are obtained by slipping between the impeller, turbine-wheel and the stator of the torque converter.
To utilize the advantages of a torque converter with respect to elasticity and torque increase for handling rapid changes in the working conditions, at the same time as the advantages of a purely mechanical transmission is utilized with respect to efficiency (in principle 100%), it has been increasingly common in working machines of today to introduce torque converters with a lock-up function. A lock-up function can provide a mechanical locking of the torque converter at a certain low degree of slipping, i.e. the gear ratio of the torque converter becomes fixed (1:1) at a certain low degree of slipping, e.g. at a low degree of slipping obtained during transportation speed. This may certainly be an alternative for wheel loaders in some specific applications.
However, the most typical application for wheel loaders is the so-called short-cycle load, in which the wheel loader moves materials between two places near to each other, e.g. moves gravel from a heap to the loading platform of a nearby truck. The transportation distances in such cycles are too short to let the torque converter reach the lock-up state. Moreover, a lock-up may not always be preferred since there is a strong interaction between the hydraulic system and the transmission line, which implies that the combustion engine benefits from the elasticity of the torque converter to reduce the interaction of the transmission line with the vehicle wheels. This is emphasized in modern wheel loaders wherein the combustion engine is utilized at lower rotational speeds due to fuel economy reasons, giving the engine even greater difficulties to recover from sudden increases in working load.
One may summarize by saying that designers would actually prefer a torque converter with a lock-up function adapted for transportation purposes. However, a lock-up function cannot be utilized in typical short-cycle loads or similar. Therefore a comparably rigid torque converter is chosen as the second best alternative. With a comparably rigid torque converter it is possible to obtain a good fuel economy both by utilizing a lower rotational speed for the combustion engine and by reducing any power consuming slipping in the torque converter. In the same way as in a rigid spring a rigid torque converter reacts less on an outer load than a soft converter. Hence, a rigid torque converter gives a reduced degree of slipping compared to a soft torque converter and the other way around; a rigid converter provides certain torque increase at a lower degree of slipping compared to a soft converter.
However, in some phases of a typical short-cycle load a much softer torque converter is preferred or even needed instead of the comparably rigid torque converter that is normally used. One such critical phase is produced when the bucket of a wheel loader is emptied on a nearby truck. Here, the bucket is usually nearly completely raised as the wheel loader approaches the truck. At the same time the hydraulic lift and tilt functions are exercised to raise the bucket even further and to finally emptying the bucket on the truck platform. In this situation it is desirable to roll slowly forward towards the truck in a controlled manner. However, a rigid torque converter will typically provide traction power for the driving wheels of such magnitude that the wheel loader rolls faster than desired even if the combustion engine is running on idle. This forces the operator of the loader to exercise the lifting, tilting and throttle controls to balance the lifting and tilting operations with the engine power (lifting and tilting may require more throttle to create the necessary power), at the same time as he has to exercise the brake to control the rolling speed. This is a rather complicated operation which lowers the productivity even for more experienced operators. In addition, this has the potential to increase the fuel consumption since the operator may choose to run the combustion engine at a higher rotation speed to meet the load from the hydraulic system while the forward rolling of the loader is controlled by the brake pedal.
Another critical phase is produced when the bucket of a wheel loader is to be filled. Naturally, it is preferred that the bucket is filled in a quick and efficient manner. This is accomplished by the operator trying to find the right balance between the bucket movement (controlled by the lifting and tilting functions) and the penetration the forward rolling controlled by the throttle pedal). Here, the traction forces from the wheels of the loader are in many situations counteracting the forces from the moving bucket i.e. the tilt and lift movements). To accomplish a quick and efficient filling of the bucket and simultaneously handling the forces from the wheels and the bucket is a more or less complicated task depending on the characteristics of the wheel loader. Here, the rigidness of the torque converter is an essential component.
Hence, both in the bucket emptying phase and in the bucket filling phase it is preferred to utilize a soft torque converter. In the bucket emptying phase this enables an improved coordination of the bucket and wheel loader movements. In the bucket filling phase this enables an improved balancing of the forces created by the bucket and wheel loader movements.
However, a soft torque converter is only preferred in such critical phases of a short-cycle load as those described above and similar. In the other phases of a short-cycle load a rigid torque converter is preferred for the reasons of performance and fuel economy.
Considering the above there is clearly a need for a working machine with a transmission line comprising a transmission unit {e.g. a torque converter) where the working machine is provided with an ability to overcome the shortcomings of known transmission units being less suitable for at least some working conditions of the working machine.
It is desirable to provide a method of the kind referred to in the introduction, which creates conditions for a more effective operation of the working machine.
According to an aspect of the present invention, a method is provided for operating a working machine provided with: a power source and a plurality of driving wheels; a working hydraulic system comprising at least one hydraulic pump powered by the power source for moving an implement on the working machine and/or for steering the working machine; a transmission line arranged between the power source and the driving wheels for transmitting torque from the power source to the driving wheels; and a transmission unit arranged in the transmission line for reducing the mechanical interaction between the power source and the driving wheels.
The method is characterized by the steps of:                detecting at least one operational parameter indicative of a working condition of the working machine,        determining if the characteristic of the transmission unit should be altered on the basis of a magnitude of the detected operational parameter,        altering the characteristic of the transmission unit if it is determined to be required.        
Altering the characteristic of the transmission unit by means of the above method provides a working machine with an improved ability to overcome the shortcomings of known transmission units being less suitable for at least some working conditions of the working machine.
This is particularly so if the working condition determines a predetermined working operation with the implement, since this constitutes a typical situation in which the need for altering the characteristic of the transmission unit can arise. Here, it may e.g. be advantageous to altering the characteristic of the transmission unit so as to reduce the mechanical interaction between the power source and the driving wheels even more leaving the driving wheels with less power and the hydraulic system with an increased power.
It is preferred that the characteristic is altered by means of at least one electric machine, since this enables a flexible and compact design. An electric machine can also be powered by means of a plurality of power sources (e.g. batteries, generators, fuel cells etc), which provides an increased freedom in the design. Moreover, electric machines react fast on commands providing an improved control over the alteration of the characteristic of the transmission unit.
It is particularly preferred that at least first electric machine is arranged downstream the transmission unit for subtracting torque from the downstream side of the transmission unit and converting this torque to electric energy. This provides energy that can be used for altering the characteristic of the transmission unit. Hence, it is not necessary to have an auxiliary power source and the requirements on a possible auxiliary power source can be relaxed. Typically, the energy should otherwise have been supplied to the driving wheels. However, the working conditions at which the characteristic of the transmission unit is advantageously altered are typically admitting that energy can be withdrawn from the driving wheels.
In addition it is preferred that at least a second electric machine is arranged upstream the transmission unit for receiving electric energy from the first electric machine and converting at least a part of this energy to torque that is added to the upstream side of the transmission unit. The use of a first and a second electric machine in this manner provides an excellent control over the alteration of the characteristic of the transmission unit.
It is also desirable to provide a working machine of the kind referred to in the introduction, which working machine enables a more effective operation of the working machine.
According to another aspect of the present invention, a working machine is provided with: a power source and a plurality of driving wheels; a working hydraulic system comprising at least one hydraulic pump powered by the power source for moving an implement on the working machine and/or for steering the working machine; a transmission line arranged between the power source (120) and the driving wheels for transmitting torque from the power source to the driving wheels; and a transmission unit arranged in the transmission line for reducing the mechanical interaction between the power source and the driving wheels.
In addition the working machine comprises:                at least one detecting unit for detecting at least one operational parameter indicative of a working condition of the working machine,        at least one control unit for determining if the characteristic of the transmission unit should be altered on the basis of a magnitude of the detected operational parameter,        at least one torque-modifying unit controlled by said control unit for altering the characteristic of the transmission unit if it is determined to be required.        
The working machine displays the same or similar advantages as the method described above.
Further advantages and advantageous features of the invention are disclosed in the following description.